The present invention relates to the field of signal processing, and more particularly, to regenerating amplitude and timing of an analog signal and to reducing propagation delay of the signal.
Manchester encoding is a well-known technique in which data and data clock components are combined into a single encoded waveform. The encoded waveform is divided into time slots of equal duration commonly called data cells during each of which one binary digit (bit) of information is conveyed. The state of the bit is indicated by a transition in the waveform occurring at the center or mid-cell point of the data cell. The direction of the transition indicates the value of the bit. At least one signal transition per data cell occurs, providing a component in the frequency domain centered at twice the bit occurrence rate.
The digital nature of the Manchester code provides the usual preservation of data information in the face of communication channel corruption by noise and other transmission effects. The timing component of the waveform eliminates the need for two physically separate transmission channels, as for example, one for data and one for timing.
Transmission standard, MIL-STD-1397B, Type E (NAVY), provides for two-way information transfer in a single bi-directional transmission channel. The transmission scheme has three states: logic 1, logic 0, and an idle state. The protocol of this standard imposes a frame format in which up to a given number of bits are transmitted as an integral unit from an information source on the transmission line. A "synch" bit begins each frame. A frame may have three or more bits in addition to the synch bit. Each frame size is variable, and the end of a frame is indicated by the presence of the idle state.
An essential element of a communication system employing the data transmission and protocol approaches just described is an element which is able to convert a Manchester-encoded waveform (normally a three-state signal) to a two level digital waveform and which can convert a two level digital waveform into a Manchester waveform. Such an element is commonly referred to as a "codec", which is a shortened form of the term coder/decoder. As is conventional, codecs are important components of data links between computers and peripherals. Frequently, such data links convey information over relatively great distances, thereby often employing a Manchester-encoded transmission channel. In such an application, a codec decodes data into a local format from the Manchester-encoded format and encodes locally-formatted data into the Manchester format. Usually, the decoder portion of the codec includes a retiming provision through which the received Manchester signal is improved by reestablishing the relative timing of the two level digital waveform transitions.
U.S. Pat. No. 5,127,023 describes a retiming decoder/encoder ("RDE") by which MIL-STD 1397 Type "E" (Low Level Serial) data transmitted is retimed and converted into a more convenient format for the purposes of switching, etc., then reconverted back to low level serial for transmission to the end user computer or peripheral device. The RDE converts low level serial signals in to a pair of complementary Manchester encoded digital signals by an input transceiver. These signals are next retimed and decoded into a simpler NRZ format by the RDE and conveyed through a switching matrix to the output section. At the output, the reverse process takes place. First, the NRZ signal is encoded back to a Manchester encoded complementary pair by the RDE encoder and sent to the output transceiver, where they are converted back into a low level serial analog signal.
While highly successful in function, the RDE described in U.S. Pat. No. 5,127,023 requires a system clock in order to synchronize the transfer of data from the input to output stages of the circuit. The circuitry to support such synchronization results in a propagation delay and a subsequent reduction in data throughput. Additionally, the system clock increases hardware complexity and EMI interference.
A paramount consideration in the distribution of low level serial data or its conveyance between two communication nodes, such as a computer and a peripheral device, is that any intermediate device be as transparent as possible to minimize propagation delay. Propagation delay of the data can interfere with the system software so as to make the transmission unworkable. As computers become faster, the data transfer rate assumes critical importance. Delay in the transmission of such data causes a reduction in the data throughput. The delay may also severely limit the communication distance between communication nodes because any delay adds to that already contributed to by cable length between equipment.
One method of reducing delay in the propagation of a data signal is to decrease the physical distance and resulting cable length between communication nodes or devices. However, there are many applications where this traditional solution is not practical. Therefore, there is a need for a system and method for minimizing the propagation delay of Manchester encoded data through an intermediate electronic device, such as a switch, in a way which maintains the integrity of the waveform (amplitude and timing) of the original encoded data.